The present invention relates to hoist mechanisms having a hoist lift line and upper and bottom sheave blocks that can become vertically misaligned.
An overhead crane includes a trolley supported by wheels for travel on elevated rails. The trolley has a frame on which a hoist mechanism is mounted. The hoist includes a rotatable hoist lift drum driven by a prime mover. A main lift line such as a rope, usually fabricated of wire, is connected to the lift drum for raising and lowering a load in response to a control actuated by an operator. The wire rope is reeved through upper and lower sheave block assemblies, and a lifting hook is secured to the lower sheave block. An operation station is often located on the crane high above the floor over which the crane travels.
For proper lifting of a load, the lower sheave block must be vertically aligned with the upper sheave block so as to be directly underneath it when the load is lifted. When hitching up a load, the rigger can easily pull the lift hook laterally and move the lower sheave block out of its desired vertical alignment with the upper sheave block. This condition is generally referred to as "side pull". If side pull exists the load will abruptly swing laterally to a plumb position when it loses contact with the floor. Side pull can result in damage to the load, to the hoist and to adjacent structure as well as possible injury to workers. The operator, from a position high above the factory floor, finds it very difficult to determine if a side pull condition exists.